whats wrong with this pu?

i have a rio grande vintage p pu (passive) in my pj bass and even now and then it will just stop working and the only way for me to get it working again is to tap somthing metal on it pretty hard then it comes back on. any ideas as to what is causing this?

You probably have a bad solder joint at the pot. Take off the pickguard and check to make sure the wire coming from the pickup is securely fastened to the pot with a good solder connection. A bad one would cause the pickup to work intermittently, as you are describing.